Solid state lighting devices are used for a number of lighting applications. For example, solid state lighting panels including arrays of solid state light emitting devices have been used as direct illumination sources, for example, in architectural and/or accent lighting. A solid state light emitting device may include, for example, a packaged light emitting device including one or more light emitting diodes (LEDs). Inorganic LEDs typically include semiconductor layers forming p-n junctions. Organic LEDs (OLEDs), which include organic light emission layers, are another type of solid state light emitting device. Typically, a solid state light emitting device generates light through the recombination of electronic carriers, i.e. electrons and holes, in a light emitting layer or region.
In many solid-state lighting applications, current is driven through an arrangement of LEDs to obtain a certain light output. This current is typically sensed to provide feedback to a controller that controls the level of illumination and/or color point. FIG. 1 illustrates a conventional technique for controlling current through one or more LEDs 110 in a lighting apparatus 100. A current sense resistor 120 is connected in series with the one or more LEDs 110. A voltage Vsense developed across the current sense resistor 120 is provided to a control circuit 140, which uses the voltage Vsense as a current feedback signal to control a transistor 130 connected in series with the one or more LEDs 110.
Current sense resistors may also be used in color point control applications. The color rendering index (CRI) of a light source is an objective measure of the ability of the light generated by the source to accurately illuminate a broad range of colors. The color rendering index ranges from essentially zero for monochromatic sources to nearly 100 for incandescent sources. Light generated from a phosphor-based solid state light source may have a relatively low color rendering index.
It is often desirable to provide a lighting source that generates a white light having a high color rendering index, so that objects and/or display screens illuminated by the lighting panel may appear more natural. Accordingly, to improve CRI, red light may be added to the white light, for example, by adding red emitting phosphor and/or red emitting devices to the apparatus. Other lighting sources may include red, green and blue light emitting devices. When red, green and blue light emitting devices are energized simultaneously, the resulting combined light may appear white, or nearly white, depending on the relative intensities of the red, green and blue sources.
In some applications, the color point of an LED lighting apparatus may be controlled by controlling currents flowing through different color LEDs of the apparatus. For example, U.S. patent application Ser. No. 12/704,730, entitled “SOLID STATE LIGHTING APPARATUS WITH COMPENSATION BYPASS CIRCUITS AND METHODS OF OPERATION THEREOF,” filed Feb. 12, 2010, describes bypass circuits configured to selectively bypass current around light emitting devices of a serially-connected string of light emitting devices to achieve, for example, color point control. Such bypass circuits may operate responsive to a voltage developed across a current sense resistor connected in series with the string of light emitting devices, such that a desired color point may be maintained in response to, for example, variations in string current caused by a dimming circuit.